Steel containers used in the beverage-can field can be broadly divided into a two-piece can and a three-piece can. The two-piece can is typified by a DrD can and a DI can, and is formed integrally by a can bottom and a body portion of the can through drawing, ironing, or bending-and-bending-back, or a combination of these workings. Steel sheets used for these can bodies include a tin plate (Sn-plated steel sheet) and a tin-free steel sheet (TFS, steel sheet having electrolytic chromic-acid treatment applied thereto), and the selection of sheets used depends upon applications or working methods.
The three-piece can is a can having a body portion and a bottom portion, which are separately formed, and is typified by a welded can having the body portion manufactured through welding. As a base material for the body portion of the can, a steel sheet having thin Sn plating or a thin Ni-plated steel sheet is used. As a base material for the bottom portion, TFS or another material is used.
For both of the two-piece can and the three-piece can, printing is applied on the external surface of the can in order to let consumers recognize the commodity value of the product. Further, the inner surface of the can is coated with resin to secure the corrosion resistance. For the conventional two-piece can, after the formation of the can, the inner surface of the can is lacquered with spray, and curved surface printing is applied to the external surface of the can. Further, recently, there is proposed a laminated two-piece can obtained by shaping a steel sheet having a PET film laminated in advance into the can (Patent Document 1, Patent Document 2). For a conventional three-piece can, the welded cans have been manufactured by welding a steel sheet having a lacquered inner surface and a printed external surface. However, there is proposed a three-piece can manufactured by using a laminated steel sheet with a PET film having printing already applied thereto, instead of using a lacquering finish (Patent Document 3, Patent Document 4).
When the two-piece can is manufactured, the steel sheet for a can is subjected to drawing, ironing and bending-and-bending-back works. Further, when the three-piece can is manufactured, the steel sheet for a can is subjected to neck forming and flange forming, and in some cases, to expand forming for the purpose of design. Thus, a laminate steel sheet used for the steel sheet for a can is required to have an excellent film adhesive property in which these works and forming can be applied without causing any trouble.
Although having excellent corrosion resistance against acid contents resulting from its sacrificial-protection effect of Sn, Sn-plated steel sheets have the outermost layer formed by brittle Sn oxide, which results in unstable film-adhesive properties. This poses a problem in that the film detaches when the above works or forming are applied, or corrosion starts from a portion having insufficient adhesive force between the film and the steel sheet.
For these reasons, a weldable Ni-plated steel sheet having excellent workability and adhesive properties is used as the laminate steel sheet for a container (Patent Document 5). The Ni-plated steel sheet has been disclosed, for example, in Patent Document 9. Although, as with the case of the Sn-plated steel sheet, the Ni-plated steel sheet may have a non-gloss surface, there is known a Ni-plated steel sheet having bright plating applied thereto through Ni-plating with a brightening agent (Patent Document 6, Patent Document 7).
However, unlike Sn, Ni does not have a sacrificial-protection effect in the acid solution. Thus, it is known that, when highly corrosive subjects such as acid beverage are contained, there occurs pitting corrosion with which corrosion advances in the thickness direction from imperfect portions such as a pinhole in the Ni-plating layer, causing a through-hole within a short period of time. Under such circumstances, there is a demand for the Ni-plated steel sheet having improved corrosion resistance. Further, there is an invention relating to a Ni-plated steel sheet having steel components adjusted such that the electric potential of the steel sheet to be plated becomes nobler to reduce the pitting corrosion (Patent Document 8).